The present invention relates to a ground detector and a DC-AC inverter for vehicles, and, more particularly, to a ground detector used preferably for detecting grounding of a DC-AC inverter for a vehicle and a DC-AC inverter for a vehicle including the ground detector.
Typically, a ground detector having a charging current calculating section, a grounding current calculating section, and a grounding fault determining section has been proposed as a ground detector for an inverter (see Japanese Laid-Open Utility Model Publication No. 63-182634). The charging current calculating section calculates the value of a charging current varied in correspondence with variation of the output frequency of the inverter. The grounding current calculating section calculates the value of a grounding current. The thus calculated value of the grounding current is compared with a setting value by the grounding fault determining section. The ground detector detects a zero-phase current using a coil.
Further, as a ground detection circuit that detects grounding of a feeder cable provided in an electric vehicle, a ground detection circuit including a circuit network and an impedance change detection circuit has been proposed (see Japanese Laid-Open Patent Publication No. 5-244701). The circuit network has a capacitor that interrupts direct current between the feeder cable and the vehicle. The impedance of the circuit network changes when grounding occurs. Such change of the impedance of the circuit network is detected by the impedance change detection circuit.
A DC-AC inverter for vehicles is employed in a vehicle for allowing use of an electric appliance inside the vehicle. Specifically, voltage of a battery of the vehicle is input to the DC-AC inverter. The voltage is then converted to a voltage at which the electric appliance is used (for example, a single-phase AC voltage of 100V or 120V) by a DC-AC conversion circuit. In the DC-AC inverter, a pair of AC output lines are DC insulated from the ground (GND) of the body of the vehicle. In a conventional case, when such insulation is lost, a leakage detector detects the fact.
Specifically, such detection is accomplished not when insulation between one of the two AC output lines and the vehicle body ground is lost but when insulation between both of the AC output lines and the vehicle body ground is lost. Thus, there is a demand that such detection be achieved when the insulation between one of the AC output lines and the vehicle body ground is lost, or grounding occurs.
The ground detector of Japanese Laid-Open Utility Model Publication No. 63-182634 needs a coil for detecting the zero-phase current. Since the coil is expensive and large-sized, a large-scaled peripheral circuit must be provided. Similarly, the method for detecting the change of the impedance caused by grounding described in Japanese Laid-Open Patent Publication No. 5-244701 involves a large-scaled detection circuit (which employs a micro-computer).